Okay, well there are two things I consider to fit under that heading...
First was 2010, when I was finishing year 13 (I'm in NZ, and while I wasn't a prefect or anything, I was on the school board of trustees), and I was walking from economics across the sports field to the buses. After a while, I'm joined by this year 7 kid...nice guy, friendly and well-meaning, if a little unwise now and then.
Enter a year 10 gentleman who has a history of bullying people. He wanders by us, trips up and falls flat on his face. Unfortunately, this coincided with said year 7 laughing at something. Of course, the year 10 boy in question becomes awfully violent and tackles him, beating the crap out of him. I step in and pull him off, and he proceeds to punch me in the back of the head to no avail as I escort my mate to his bus. The year 10 lad gets on too, and gets together with his friends to try to get me at my stop (the foot of my drive). Against one year 10, I was confident, but against three who actually knew how to fight, I was less so.
Fortunately, the bus driver got wind of it, and as he dropped me off he closed the doors as they tried to follow, and the bus drove off. That year 10 got banned from the bus for a month because of his stupidity, and though his dad tried to threaten me in my own home soon after, eventually he had the good grace to gtfo. I'm still really proud of that, to be honest.
The second event took place last year. I've since started university, and living in the city has been quite different from what I'm used to. However, a few weeks ago I came across an unfortunate scene indeed...this rather angry woman who was beating up her boyfriend because, and I quote "he didn't get me the bracelet I wanted." This was a bit trickier to handle, because I know that there's a lot of fuss about whether a man is allowed to defend himself, or someone else, against a woman. It just gets taken badly. Eventually, however, I pull her off him, and offer to buy the guy a drink. Didn't hear from him again after that. It just looked like he needed a break, bless him.